This study was designed to compare and identify the adducts formed between rosmarinic acid and amino acids at two different modification conditions. Covalent interactions between different amino acids, including [L-Lysine (L-Lys) and its derivative,tert-butyloxycarbonyl-L-lysine (N-Boc-Lys) and L- cysteine (L-Cys)] and rosmarinic acid (Ros) were conducted with two different methods; alkaline (pH 9) and enzymatic [in the presence of tyrosinase (polyphenoloxidase , PPO)]. The formed adducts were identified and characterized using flow injection electrospray ionization - quadrupole time of flight (FI- ESI-QTOF) mass spectrometry. Many adducts were identified for the first time. The obtained data exhibited slight differences between the adducts formed after alkaline and enzymatic treatment. In case of the incubation of Ros with both methods without amino acids, numerous adducts, such as Ros quinone, Ros dimer and decarboxylated Ros at [M−H] - with an m/z of 359.08, 719.17 and 313.1 Da, respectively, were identified .Otherwise, in the presence of amino acids, the monomer and dimer of Ros were covalently added to the side chains of selected amino acids. Additionally, the covalent attachment between oxidized Ros and the side chains of two molecules of amino acids was also identified. Finally, L- Cys was the most susceptible amino acid to react with oxidized Ros compared to L- Lys and N-Boc-Lys. It could be concluded that, the results show some similarities between the adducts formed at both modification conditions. Moreover, the covalent interactions between Ros and the side chains of molecules of lysine or cysteine may also be expected to occur in proteins.